The Rocket Equation

A rocket moves forward by pushing propellant out in the opposite direction. It can't pull its way through the air like an airplane can do with its propellers. It can't push its way along like a car does with its tires.
By pushing propellant out at a high velocity, a rocket accelerates. The velocity of the propellant is a measure of the efficiency of the engine and is called the Specific Impulse or the Effective Exhaust Velocity.
We can calculate the ratio of fuel to payload using the Tsiolkovsky rocket equation.
initial mass / final mass = mass ratio = exp (delta-v / exhaust-velocity)
To reach low earth orbit from Earth, it requires a change in velocity of about 10 kilometers per second. The effective exhaust velocity of Hydrogen-Oxygen engines can be up to 3 kilometers per second. Applying the rocket equation, yields a mass ratio of 28.0.
Bottom line: To launch 1 kg into low earth orbit requires 27.0 kg of rocket fuel.

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